The present invention relates to polythioethers and, more particularly, to polythioethers having a specified ratio of constituent atoms therein and a process for the formation thereof.
Thiol-terminated sulfur-containing polymers have a long history of use in aerospace sealants because of their fuel resistant nature upon cross-linking. Among the commercially available polymeric compounds having sufficient sulfur content to exhibit this desirable property are the polysulfide polymers described, e.g., in U.S. Pat. No. 2,466,963 and sold under the trade name LP(copyright) polysulfide by Morton International of Chicago, Ill. and the alkyl side chain containing polythioether polymers described, e.g., in U.S. Pat. No. 4,366,307 that are sold in complete sealant formulations by PRC-DeSoto International, Inc. of Glendale, Calif. In addition to fuel resistance, polymers useful in this context must also have the desirable properties of low temperature flexibility, liquidity at room temperature, high temperature resistance, a reasonable cost of manufacture, and not be so malodorous as to prevent commercial acceptance of compositions that contain the subject polymers.
U.S. Pat. No. 4,366,307 teaches the use of hydroxyl-functional thioethers having pendent alkylene groups to obtain polymers having good flexibility and liquidity. However, the disclosed condensation reaction has a maximum yield of about 75% of the desired condensation product. Furthermore, the acid-catalyzed reaction of beta-hydroxysulfide monomers, such as thiodiglycol, yields significant quantities (typically not less than about 25%) of an aqueous solution of thermally stable and highly malodorous cyclic byproducts, such as 1-thia-4-oxa-cyclohexane. As a result, the commercial viability of the disclosed polymers is limited. Further, pendent alkylene chains increase the carbon content of the polymer necessitating a high sulfur content to achieve sufficient chemical resistance properties.
U.S. Pat. No. 5,959,071 teaches the use of pendant alkylene chains and high sulfur content to achieve the chemical resistance and room temperature liquidity required for aerospace sealant formulations.
Certain prior art work has developed hydroxyl-terminated polythioethers by condensing thiodiglycol in the presence of certain etherifying catalysts as, for example, shown in U.S. Pat. Nos. 3,312,743 and 3,335,189. Compounds produced by these patents give semi-crystalline waxy solids, gums or low molecular weight liquids that have limited commercial utility.
Another desirable feature in polymers suitable for use in aerospace sealants is high temperature resistance. Inclusion of covalently-bonded sulfur atoms in organic polymers has been shown to enhance high temperature performance. However, in the polysulfide polymers disclosed in U.S. Pat. No. 2,466,963, the multiple xe2x80x94Sxe2x80x94Sxe2x80x94 linkages in the polymer backbones result in compromised thermal resistance. In the polymers disclosed in U.S. Pat. No. 4,366,307, enhanced thermal stability is achieved through replacement of polysulfide linkages with polythioether (xe2x80x94Sxe2x80x94) linkages. In practice, however, the disclosed materials have compromised thermal resistance due to traces of the residual acid condensation catalyst.
U.S. Pat. No. 5,912,319 teaches the use of combinations of certain polythiols with oxygenated dienes resulting in polythioether polymers that are liquids at room temperature and pressure and have desirable physical properties. Further, these combinations are substantially free of residual catalysts and malodorous cyclic byproducts. Unfortunately, the oxygenated dienes described are very difficult to prepare and only a limited number of commercial compounds are known to exist.
In addition to the foregoing deficiencies with the previously known polythioethers, the prior art polythioethers are typically also crystallizing products which, even if liquid or semi-liquid at ambient temperatures, when cooled sufficiently to solidify will not return to their previous liquid state even when the temperature is raised to ambient.
A polythioether comprising:
R4xe2x80x94S"Brketopenst"R1xe2x80x94Sxe2x80x94CH2CH2xe2x80x94(R2)mxe2x80x94S"Brketclosest"nR1SR4
wherein R1 is a C1-10 alkyl, xe2x80x94(R3Q)pR3xe2x80x94 or C6-C20 aryl where Q is O or S, each R3 is independently C1-6 alkyl, and p is an integer between 0 and 6; R2 is C1-6 alkyloxy or C5-12 cycloalkyloxy, R4 is H, C1-6 alkyl, C1-6 alkyl alcohol and C0-6 alkyl substituted with "Brketopenst"CH2CH2(R2)m"Brketclosest"X, where X is a halogen, m is an integer between 1 and 4, and n is an integer selected to yield a molecular weight for said polythioether of between 1000 and 10,000 Daltons. A polyfunctionalizing agent is optionally provided in order to increase the functionality of a polythioether from 2 to 4 with the most preferred range being 2.05 to 3.00.
A process for forming such a polythioether includes the step of reacting a polythiol with a polyhalide in the presence of an aqueous base. The use of a polythioether is contemplated as an aerospace sealant.